(1) Field of the Invention
The present invention relates to a thin film thermistor element (a thin film NTC thermistor element) for use in temperature sensors of a variety of equipment such as information processing equipment, communication equipment, housing-facility equipment, automobile electrical equipment, and to a method for the fabrication thereof.
(2) Description of the Related Art
An NTC thermistor element of oxide semiconductor material as an element for the detection of temperature is typically constructed by formation of an electrode (e.g., an electrode of Ag) on an end face of an oxide sintered body chip whose major component is a transition metal such as Mn, Co, Ni, and Fe and which has a spinel type crystal structure, by means of application or baking.
Such NTC thermistor elements have the following advantages over thermocouples and platinum resistance temperature sensors. Therefore, the NTC thermistor element has currently been in wide use.
(1) The resistance temperature change is great, allowing high temperature resolution;
(2) Determination can be carried out with a simple circuit;
(3) Formed of material which is relatively stable and unsusceptible to the influence of ambient conditions, achieving less deterioration with time, being superior in reliability; and
(4) Mass production is possible, holding down costs.
Further, the NTC thermistor element is used not only to measure the temperature of an object but also to control a current in a power supply device. The NTC thermistor element has the property that its resistance value is high at room temperature but decreases as the temperature rises. Because of such a property, the NTC thermistor element serves, for example, in a switching power supply, as an excessive current control element which controls an excessive current (i.e., an initial rush current) that starts flowing the instant the power supply switch is turned on and which thereafter becomes low in resistance with the rise of temperature by self exothermicity, whereby the loss of power is held low in the steady state. NTC thermistor elements that find their way into such an application are fabricated from, for example, rare earth transition metal oxide as a thermistor material. More specifically, a sintered body of lanthanum cobalt oxide having a perovskite type crystal structure is used, wherein a thin film electrode of silver is formed atop the sintered body by means of sputtering (see Japanese Unexamined Patent Gazette No. H07-230902).
Apart from the above, recently, with the reduction in size and weight of electronic equipment and with the improvement in performance of same, there have been strong demands for the ultra-miniaturization of thermistor elements in element size (for example, below 1 mmxc3x970.5 mm) as well as for the high accurization of the resistance value and the B constant, i.e., the resistance change-rate with respect to temperature, at measuring temperatures (for example, a variation of 3% or below). However, due to some processing problems, difficulties will arise when considerably down-sizing such a thermistor element comprising an oxide sintered body. In addition, there is created the disadvantage that, as thermistor elements are down-sized, both the resistance value and the B constant undergo greater variation because of the problem of processing accuracy.
In order to cope with such problems associated with thermistor elements using oxide whose major component is a transition metal, such as Mn, Co, Ni, and Fe, having a spinel type crystal structure, the development of thin film thermistor elements employing thin film technology for the formation of thermistor material and electrodes has now been popular. This type of thin film thermistor element is fabricated as follows. A thermistor thin film is formed by a sputtering technique targeting on a sintered body of complex oxide of, for example, Mn, Ni, Co, and Fe, which is followed by formation of a predefined electrode pattern on the thermistor thin film. However, such a thermistor thin film formed by sputtering suffers several problems. First, it is unlikely to obtain good crystallinity. Second, the stability is low, therefore resulting in causing both the resistance value and the B constant to undergo considerable variation with time. The particular problem is that high temperature durability is low. As to this problem, a technique has been known in the art, in which a thermistor thin film formed by sputtering is subjected to heat treatment in air at, for example, from 200 to 800 degrees centigrade for crystallization to have a spinel type structure (see Japanese Unexamined Patent Gazette No. S63-266801, Japanese Unexamined Patent Gazette No. H03-54842, and xe2x80x9cYashiro Institute of Technology Transactionsxe2x80x9d Vol. 8, pp. 25-34, by Masuda and others).
However, in the case such a thermistor thin film of spinel type oxide semiconductor formed by sputtering is crystal grown by heat treatment, it is likely that the variation in crystal grain diameter in the resulting polycrystalline substance is great. Because of this, even with regard to thermistor elements of the same fabrication lot, they vary considerably in electrical characteristic, e.g., the resistance value and the B constant. Moreover, even if heat treatment is carried out at, for example, 400 degrees centigrade or above, this will find difficulties in improving stability to a greater extent, and it is also difficult to improve high temperature durability.
Bearing in mind the above-described points, the present invention was made. Accordingly, an object of the present invention is to provide a thin film thermistor element capable of holding, for example, the variation in resistance value low for the achievement of high accuracy and capable of improving high temperature durability for the achievement of high reliability, and a method for the fabrication of such a thin film thermistor element.
In order to achieve the above-described object, the present invention provides a thin film thermistor element. The thin film thermistor element of the present invention comprises a thermistor thin film and a pair of electrodes formed on the thermistor thin film, wherein the thermistor thin film has either a spinel type crystal structure which is oriented mainly in a (100) surface, a bixbite type crystal structure (particularly, a bixbite type crystal structure which is oriented mainly in a (100) or (111) surface), or a rhombohedral perovskite type crystal structure (particularly, a rhombohedral perovskite type crystal structure which is oriented mainly in (012). A thermistor thin film having a spinel type crystal structure with a (100) surface orientation or bixbite type crystal structure can be formed of, for example, a thin film of oxide whose major component is manganese. Further, a thermistor thin film having a rhombohedral perovskite type crystal structure can be formed of, for example, a composition containing lanthanum cobalt oxide. Furthermore, it is preferred that a thermistor thin film having a spinel type crystal structure with a (100) surface orientation has a crystal grain which has grown by crystallization into a columnar shape in a direction perpendicular with respect to the thermistor thin film.
The above-described thermistor thin films of the present invention each show less variation in the crystal grain diameter in comparison with thermistors of a sintered body and thermistor thin films having a no-orientation spinel type crystal structure, because of which the variation in electrical characteristic (such as the resistance value and B constant (i.e., the change rate of resistance to temperature) can be held low and, in addition, the crystal state is relatively stable so that the deterioration with time of such electrical characteristics can be held low and the high temperature durability is high. Accordingly, with such a crystal structure, it becomes possible to achieve high-accuracy, high-reliability thermistor elements. Further, formation is carried out through the use of thin film technology, whereby down-sizing is easier to achieve in comparison with the case where a sintered body thermistor is employed.
Thermistor thin films of the type described above can be formed by alternately carrying out a film formation step by, for example, sputtering and an anneal step. More specifically, an arrangement is made, wherein at least either one of a substrate holder for holding a backing substrate and a target placed face to face with the substrate holder is rotated and wherein the backing substrate is held at a position eccentric from the center of the rotation in the substrate holder while the target is covered with a shield cover so that a part of a position eccentric from the rotational center in the target is exposed, whereby the film formation step by sputtering can be carried out on the backing substrate at a rotational position whereat the backing substrate faces the exposed portion of the target while on the other hand the anneal step can be carried out at a rotational position whereat the backing substrate faces the position of the target covered with the shield cover. Further, it is possible to form a higher-accuracy, higher-reliability thermistor element by performing a heat treatment after the formation of a thermistor thin film of the type described above, wherein the substrate temperature and the heat treatment temperature during the film formation by sputtering are set to various values according to the composition and the film formation time of a thermistor thin film that is formed. For example, a film formation step is carried out with a substrate heated to 200-600 degrees centigrade and a heat treatment is carried out in air at 600-1000 degrees centigrade, whereby the foregoing thermistor clement can be fabricated easily. If the thermistor thin film formation is carried out in an atmosphere in which the rate of flow between argon gas and oxygen gas is 3 or greater, this relatively facilitates formation of a thermistor thin film having a spinel type crystal structure with a (100) surface orientation, and if the heat treatment is carried out at 1100 degrees centigrade or below, this relatively facilitates formation of a thermistor thin film having a bixbite type crystal structure.
Moreover, in the above-described thin film thermistor element, an electrode is provided with a trimming portion for the adjustment of resistance, and the trimming portion is cut using laser light irradiation or the like to make a resistance adjustment, whereby it becomes possible to facilitate the fabrication of higher-accuracy thin film thermistor elements.